1. Field of the Invention
The present invention relates to prosthetic devices, and more specifically, it relates to an artificial spinal disc for replacement of a natural spinal disc.
2. Description of Related Art
The vertebral spine is a complex arrangement of many structures, with many areas of specially cushioned apposition. The vertebral bones are twenty-four in number, not including the sacrum, and they gradually vary in size, shape and load distribution from the cervical to the thoracic to the lower lumbar vertebrae. The vertebrae, amazingly, are very different between the first cervical and the last lumbar vertebra. Nonetheless, the bony vertebral bodies of the spine are each separated by a relatively soft intervertebral disc that acts as a joint, allowing flexion, extension, lateral bending and axial rotation. Fibrous tissues, emulating scar tissues, may act somewhat similarly to the bonding elements that make up the ligaments of the spine, as well as the outer portions of the relatively soft intervertebral discs. If a synthetic vertebral disc were to be placed to repair one that is damaged, it would be beneficial to have the participation of these fibrous fixing elements in a relatively controlled and maximized fashion.
The typical vertebra has a thick interiorly located bone mass called the vertebral body (with a neural vertebral arch that arises from a posterior surface of the vertebral body). The intervertebral disc primarily serves as a mechanical cushion between the vertebral bones, permitting controlled motion within the vertebral segments of the.axial skeleton. The normal disc is a unique mixed structure, comprised of three component tissues, including the nucleus pulposus (nucleus), the annulus fibrosus (annulus), and the two opposing vertebral end plates. The two vertebral end plates are each composed of thin cartilage overlying a thin layer of hard cortical bone, which attaches to a spongy, richly vascular cancellous bone of the vertebral body. The vertebral end plates thus serve to attach the adjacent vertebra to the disc. In other words, a transition zone is created by the end plates between the malleable disc and the bony vertebra.
The annulus of the disc is a tough outer fibrous ring that binds together the adjacent vertebrae. The fibrous portion is much like a laminated automobile tire measuring about 10 to 15 mm in height and about 15 to 20 mm in thickness. Fibers of the annulus consist of 15 to 20 overlapping multiple plies and are attached at the superior and inferior vertebral body at a roughly 30-degree angle in both directions. This configuration particularly resists torsion, as about half of the angulated fibers will tighten when the vertebrae rotate in either direction relative to each other.
Inside the annulus there is a relatively liquid core, the nucleus. The healthy natural nucleus has a high water content and aids in the load bearing and cushioning properties of the spinal disc; however, the spinal disc may be displaced or damaged due to trauma or disease. A disc herniation occurs when the annulus fibers are weakened or torn and the nucleus becomes permanently stressed, extended or extruded out of its normal internal annular confines. A herniated or slipped nucleus can compress a spinal nerve posteriorly, resulting in pain, loss of muscle control or even paralysis. Alternatively, in disc degeneration the nucleus loses its water binding capacity and deflates as though the air had been let out of a tire. Subsequently, height of the nucleus decreases, causing the annulus to buckle in areas where the laminated plies are loosely bonded. As the overlapping laminated plies of the annulus begin to buckle and separate, either circumferential or radial annular tares may occur and contribute to persistent and disabling pain. Adjacent ancillary spinal facet joints to the rear may also be forced into an overriding position, which may cause additional back pain as tissues are damaged due to irregular contact and force application.
Upon identification of the abnormality causing the conduction disorders, surgery may be required to correct the problem if more conservative treatment fails. For those problems associated with the formation of osteophytes or herniations of the intervertebral disc, one such surgical procedure is intervertebral discectomy. In this procedure, the involved vertebral bodies are exposed and the invertebral disc is removed, thus removing the offending tissue or providing access for the removal of the bone osteophytes. A second procedure, termed a spinal fusion, may then be required to fix the vertebral bodies together to prevent movement and maintain the space originally occupied by the intervertebral disc. Some minor loss of flexibility in the spine may result, but because of the large number of vertebrae the loss of mobility is usually acceptable.
During spinal fusion following a discectomy, an implant is inserted into the intervertebral space. This intervertebral implant is often a bone graft removed from another portion of the patient""s body, termed an autograft. The use of bone taken from the patient""s body has the important advantage of avoiding rejection of the implant, but has some shortcomings. There is always a risk in opening a second surgical site for obtaining the bone graft, which can lead to infection or pain for the patient, and the site of the bone graft is weakened by the removal of bony material. The bone implant may not be perfectly shaped and placed, leading to slippage or absorption of the implant, or failure of the implant to fuse with the vertebrae.
Other options for a graft source for the implant are bone removed from cadavers, termed an allograft, or from another species, termed a xenograft. In these cases, while there is the benefit of not having a second surgical site as a possible source of infection or pain, there is the increased difficulty with graft rejection and the risk of transmitting communicable diseases.
An alternative approach to using a bone graft is to use a manufactured implant made of a synthetic material that is biologically compatible with the body and the vertebrae. Several compositions and geometries of such implants have been utilized, ranging from simple blocks of material to carefully shaped implants, with varying success. No fully satisfactory implant has been reported. In some instances, the implanting surgery is readily accomplished, but the results are unsatisfactory due to side effects or dislocation of the implant In other instances, the implant requires a complex surgical procedure that is difficult to perform and still may not lead to correction of the problem for the reasons indicated.
In U.S. Pat. Nos. 5,306,309 and 5,683,464 by Wagner et al., the authors provide a solid body spinal disc implant and surgical implantation kit.This solid body implant is made of a biocompatible synthetic material designed to engage the cortical bone region of the vertebrae after implantation. This type of implant does not address the central portion of the vertebral body region made of cancellous bone. It would be advantageous to have an implant with a central layer similar to this more resilient and less dense type of bone. A multi-layered design that mimics the mechanical properties of a natural spinal disc is desirable.
U.S. Pat. No. 5,123,926 by Pisharodi discusses a disc prosthesis composed of biologically compatible material. This prosthesis could be implanted and expanded to conform to the vertebral space so as to replicate a natural disc""s function. Expansion would be achieved by injecting a liquid or gas substance through a port into the disc prosthesis. While this would provide a tight fit in the disc space, problems could arise should the prosthesis rupture.
There is a need for a truly stable yet fully flexible artificial spinal disc, which could be utilized, in a surgical procedure with a high probability of success without producing undesirable side effects. The present invention fulfills this need, and further provided related advantages.
It is an object of the present invention to provide a multi layered artificial spinal disc that can be used to replace a damaged natural spinal disc. This and other objects of the invention will be apparent from the teachings of the present invention.
The artificial spinal disc of the present invention is comprised of biocompatible materials that when implanted will respond similarly to a natural disc. Using a computed tomography (CT) scan, ultrasound imaging, and/or magnetic resonance imaging (MRI) the artificial disc can be shaped to meet a patient""s specific needs. Through imaging, the artificial disc can be contoured to fit the bony irregularities present in the spine to produce a tight fit and provide for maximum bone to implant bonding.
The implant is designed to approximate the size and shape of a natural vertebral disc. This implant is comprised of three distinct layers. The top and bottom layers are made of bone permeable material such as porous titanium. Use of bone permeable materials promotes permanent bonding between the artificial disc and the spine bone. The central layer of the implant is comprised of biocompatible polymers that mimic the mechanical properties of natural discs. To further duplicate the mechanical properties of natural discs the middle portion of the central layer can be composed of softer material (e.g., silicon rubbers and/or polyurethane/silicon composites) and surrounded by the central layer of biocompatible polymer.
The present invention provides a strong bond between the different layers of the implant. This can be achieved by pressure injection of the polymer into the porous titanium. The top and bottom layers of the implant can also be machined with precision cutting machines or lasers to produce trapping structures thinly coated with porous titanium to promote bone in-growth between the implant and the spine.
The thickness of the hard top and bottom layer of the implant can be uniform or vary in thickness to modify compressibility of the disc. During the surgical procedure, it may be necessary to mechanically scrape the vertebral bone to produce a clean planar surface and cause a strong vertebral bone-to-implant bond to form. To increase bonding, bone growth factors can be applied to the implant or vertebral bone during surgery. Following surgery, motion may need to be limited to allow time for proper bonding to occur.
The invention provides an artificial spinal disc with a lip contoured into the hard top and bottom layer. The lip on the implant can be packed with a paste form of artificial bone that fills in gaps between the implant and the vertebral bone. The lip also limits lateral motion and accurately locates the disc.
An alternative embodiment of the present invention has an annular region on the top and bottom bone permeable surface designed to bond to the hard cortical bone of the vertebra. To produce the desired mechanical properties, the core of the implant is comprised of compressible biocompatible polymer. The compressible core is held in place by a high strength, less compressible outer support made of polymers or composites. This outer support prevents excessive movement of the outer surface that could put pressure on the spinal cord or nerves. Carbon fibers can be embedded in the composite material and by varying the orientation of the fibers; it is possible to duplicate the properties of the natural disc annulus fibrosus.
Other objects and advantages of the present invention will become apparent from the following description and accompanying drawings.